Use of the iBSS
The iBSS bootstraps the iBEC, which prepars and executes the Restore Ramdisk. in addition, it sends messages to iTunes on the restore to supervise the restore process. It also integrity checks the images uploaded, and on iOS5+ os the image responsible for APTicket, by uploading the nonce string to iTunes then checks for the match of the APTicket and nonce and the signatures on APTicket. On custom firmwares, the iBSS is patched out of every signature check, but on certain corcumstances it still generates nonce. Check APTicket for further detail.